Red Heifer Sprinkling and Missing a Day

Question

Numbers 19 discusses administering of the Red Heifer's ashes by way of sprinkling water. People who become impure from contact with the dead must be sprinkled by the water on the third and seventh days from contraction of impurity, or they remain impure, (verse 12). Is their any recourse for someone who just couldn't get a hold of a pure person and the water on those specific days?

Answer 1

Rambam Parah Adummah 11:2 (English)

מי שנטמא במת, ושהה כמה ימים בלא הזיה--כשיבוא להזות, מונה בפנינו שלושה ימים; ומזין עליו בשלישי ובשביעי, וטובל בשביעי ומעריב שמשו.‏

במה דברים אמורים, בעם הארץ שבא להזות, שאפילו אמר היום השלישי שלי--אינו נאמן, שמא היום נטמא; לפיכך צריך למנות בפנינו. אבל חבר שבא להזות, מזין עליו או על כליו מיד.‏

מי שהוזה עליו בשלישי, ולא הוזה בשביעי, ושהה כמה ימים--טובל בכל עת שירצה אחר השביעי, בין ביום בין בלילה, ומזין עליו ביום, בין קודם טבילה בין אחר טבילה: אפילו טבל בליל תשיעי, או ליל עשירי. ומזין עליו למחר, אחר הנץ החמה..‏

When a person became impure due to a corpse and remained several days without having the water sprinkled upon him, when he comes to have the water sprinkled upon him, he should count three days in our presence. The water is sprinkled upon him on the third and seventh days. He immerses in a mikveh on the seventh day and waits until nightfall.

To whom does the above apply? To a common person who comes to have the water sprinkled upon him. Even if he says that this is the third day after he contracted impurity, his word is not accepted, for perhaps he became impure on this day. Therefore, he must count the days in our presence. If, by contrast, a chaver comes to have the water sprinkled upon him, it may be sprinkled on him and his implements immediately.

When such water was sprinkled on a person on the third day, but it was not sprinkled upon him on the seventh day, he may immerse in a mikveh whenever he desires after the seventh day, whether during the night or during the day and the water should be sprinkled on him during the day, whether before immersion or after immersion. Even if he immersed on the night of the ninth day or the night of the tenth day, the water may be sprinkled on him on the following day after sunrise.

Essentially, it just has to be at least that number of days.

(I'll note there is some controversy about delaying the second sprinkle based on Kiddushin 62. The above is the Rambam's ruling. Check with your local rabbi for practical guidance.)